MIRI Imaging Recommended Strategies

Recommendations for planning most MIRI imager science observations, based on pre-launch knowledge of the instrument. 

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The MIRI Imager offers 9 broadband filters covering wavelengths from 5.6 to 25.5 μm (Bouchet et al. 2015). Observers should follow the mode-independent general guidelines described for MIRI. Additional information is available in the MIRI Cross-Mode Recommended Strategies and MIRI TSO Recommended Strategies articles.



Detector readout mode

See also: Understanding Exposure TimesMIRI Generic Recommended Strategies (Detectors)

The default readout mode for imaging is FAST*FAST mode is mandatory for subarray exposures. FULL array observations may use SLOW mode, primarily to limit data volume when MIRI is used in parallel.


* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.



Dithering

See also: MIRI Imaging Dithering,MIRI Imaging APT Template

For most science cases, dithering is a highly recommended and necessary practice for the following reasons:

  • Allows good PSF sampling—this is mostly relevant when using the F560W filter; the MIRI imager Nyquist-samples the PSF for wavelengths ≥ 6.25 μm
  • Minimizes detector cosmetics and defects
  • Makes possible accurate background measurements for point sources, and at longer wavelengths permits tracking of potential telescope thermal emission variations
  • Mitigates the impact of bad pixels
  • Allows tracking detector drifts at the timescale of the dwell time per dither position (i.e., total length of time the telescope exposes at a dither position)

The Astronomer's Proposal Tool (APT) offers a set of pre-defined dither patterns for imaging. Photometric time-series observations may make use of a no-dither option. There are 2 main aspects to dithering: (1) choosing an adequate pattern and (2) deciding the dwell time (i.e., how long to stay integrating at a single dither position). 


Choosing a dither pattern  

The user has to select a dither pattern that ensures enough redundancy, hence good quality, in the data. Below is the list of MIRI imaging dithers offered by the APT with usage recommendations.


Table 1. MIRI imaging dithers offered by the Astronomer's Proposal Tool

Dither patternApplicabilityCharacteristicsUsage
4-Point-SetsBroad

Choices of point and extended sources; good PSF sampling in all subarrays/bands

Allows offsets to accommodate multiple exposures

Optimized dither solution for most science cases
CYCLINGBroadGaussian-distributed cloud of pointsGood dither solution for many science cases. The user has to define the pattern by choosing the points within a cloud.
2-PointLimitedTwo separated exposures; Allows for simple background subtraction

The use of this pattern will have to be justified, both in terms of science use and data quality.

A use example: shallow mosaic where each tile has larger separation than 20”; each mosaic position could perform a 2-pt dither, ensuring both good redundancy and larger coverage.

REULEAUXSpecialized12-point dither in different sizes to fit subarraysSpitzer users may be familiar with this pattern. Categorized as specialized because it is complex to design observations that are optimized for its use.
Coordinated ParallelsParallel observations2-, 3-, 4-, and 9-step, filter-dependent patternsThe patterns optimize pixel phase sampling for both the prime and parallel instrument modes. If a pattern is not available for a given MIRI filter, using the pattern for a filter at the next longer central wavelength is recommended.


Dwell time limit

Dwell time is defined as the length of time spent at a single dither position. Since multiple exposures are discouraged at a single dither position, the dwell time should also define the exposure length. For the long wavelength filters, the dwell time is limited to the amount of time it takes to reach a ceiling in the signal-to-noise ratio (SNR) due to high background levels. This is known as the dwell time limit. The following table gives recommendations on the length of time observers should spend at a single dither position (i.e., exposure length). These estimations are based on ground measurements of flight-like detectors. 


Table 2. Recommendations on the length of time at a single dither position (exposure length)

MIRI filterBackground typeLimitation in dwell time?Recommendation
F560W to F1800WLowNo

If possible, a minimum of at least 40 groups in FULL/FAST mode (111 s)  will minimize the effects of drifts at low backgrounds.  A maximum of 360 groups in FULL/FAST is recommended (although not required).

F560W to F1800WMediumNoA minimum of 5 groups and a maximum of 1,000 s integration is recommended  (but not mandatory)
F2100W and F2550WAlways highYes

Maximum dwell time of 8 minutes; longer exposures will reach a ceiling in the SNR.


Guidelines on the exposure length can be also found in the MIRI Cross-Mode Recommended Strategies article. Users should also note that the observatory imposes a limit of 10,000 s on the length of an individual exposure to allow for moves of the high gain antenna (HGA). This is only waived for TSO observations.



Target acquisition

See Main Article: MIRI Cross-Mode Recommended Strategies (Target Acquisition)

Target acquisition (TA) is currently not being offered for the MIRI imager. Given that the expected pointing accuracy of the observatory, most imager science use cases will not need target acquisition. Users interested in imaging TSO science may wish to have TA capabilities for at least the SUB64 subarray, but it is currently not supported.



Filter ordering

At longer wavelengths, the MIRI imager data will be affected by an additional high background component coming from the telescope emission that can potentially imprint latents in the detector. To avoid persistence due to transitions from high to low backgrounds, it is best to sort the imager exposures from short to long wavelengths. The observer has control of the order in which the filters are used: it is the same one that is specified in the MIRI Imaging Template APT at the time of proposal submission.



Background observations

See also: JWST Background Model, Background-Limited ObservationsMIRI Generic Recommended Strategies (Background), APT Targets

The recommendations for obtaining background observations for MIRI imaging can be found in the general background information for MIRI.

It is important to note that when an observation program assigns a background to a science target, that creates a formal association between them. By doing this, the pipeline will automatically subtract the background exposure from the target exposure. To avoid having undesired residuals from this step, the user should:

  • Choose a background area that is as clean as possible of sources. Given the MIRI sensitivity, it is unlikely to find "empty" regions in the imager FOV (about 74" × 113" in FULL array).

  • Dither the background target. By doing that the pipeline will stack the dithered images thus removing unexpected sources, and use that combination to remove the background from the science data. Observers should carefully consider how many dither points will be needed to achieve the required SNR in their background, and to remove sources present in the background region with the stacking technique.


References

Bouchet et al., 2015, PASP, 127, 612B
The Mid-Infrared Instrument for the James Webb Space Telescope, III: MIRIM, The MIRI Imager

Glasse et al., 2015, PASP, 127, 686G
The Mid-Infrared Instrument for the James Webb SpaceTelescope, IX: Predicted Sensitivity




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